1. Field of the Invention
The present invention relates to sensors, and more specifically, to a microstrain sensor usable to measure deformation, acceleration, tension, and the like, in any application but particularly in the context of vehicle safety.
2. Description of Related Art
A control system is a system in which the operation of at least one device is to be controlled based on some parameter related to the system. In any control system, at least one sensor is utilized to gather data about a parameter of interest, and transform the data into a form readable by the system. Thus, any sensor is also a transducer. Typically, a sensor provides an electrical signal in which some characteristic such as the amplitude or frequency of the voltage varies in proportion to the parameter of interest. Such signals can be gathered from multiple sensors and processed by a computer to provide a control value for the device to be controlled.
One type of sensor is a strain gauge. A strain gauge is any sensor that deforms with an object to measure the object""s strain, or deformation. The magnitude of the deformation can be useful in performing stress and structural analysis and the like, or for indirectly obtaining some other value of interest. For example, through manipulation of the strain gauge or the member on which the gauge is mounted, the strain gauge can be used to indirectly measure other parameters, such as the mass of an object attached to the member, the acceleration of the member, or the like.
Typically, strain gauges have one or more resistors for which the resistance changes according to the configuration of the resistor (i.e., a sensing resistor). Four total resistors are normally linked together in a diamond configuration to form a circuit known as the Wheatstone Bridge. The diamond configuration forms two separate current paths along which an input current can travel. A signal detector, such as an ammeter or voltmeter, straddles the two current paths so that current or voltage between the two paths can be measured. When resistance along one path increases, current can be expected to move through the signal detector to reach the other, lower-resistance path. Such an arrangement enhances the sensitivity of the sensor because the output signal is not proportional to the absolute resistance of the sensing resistor, but is proportional to the change in resistance between the current paths.
For example, in a quarter-bridge circuit, one of the four resistors may be a sensing resistor attached to the member in such a fashion that the resistor lengthens or shortens when the member deforms. The sensing resistor may take the form of a thin, meandering, conductive strip mounted to a thin piece of insulative plastic or ceramic. The sensing resistor may be attached to the beam by an adhesive. If vertical bending of a beam is to be measured, the sensing resistor may be affixed to the top or bottom surface of the beam so that the sensing resistor lengthens or shortens when the beam bends or relaxes.
The output voltage of the circuit may be measured to determine how far the sensing resistor is deflected. In the alternative, one of the other three resistors may be a variable resistor (i.e., a resistor with adjustable resistance). The resistance of the variable resistor may be adjusted until the bridge is balanced, i.e., the resistance change of the sensing resistor has been fully compensated for so that there is no output voltage. The resistance value of the variable resistor may then be read to determine by inference what the resistance of the sensing resistor must be.
Half-bridge and Full-bridge type circuits are also commonly used. A half-bridge circuit has two sensing resistors. The sensing resistors may be arranged in additive fashion, in which case they are both placed on the same side of the beam to receive the same deformation. If the sensing resistors are placed side-by-side, the effect is to negate the influence of lateral bending on the vertical bending measurement obtained by the sensor. The sensing resistors may alternatively be arranged in subtractive fashion and positioned on opposite sides of the beam (for example, one on the top side and one on the bottom side) so that the deformation they receive is opposite. The effect of such placement is to negate axial strain such as tension or compression along the length of the beam. In such a way, a half-bridge circuit can be used to remove undesirable strain effects from the pure vertical bending output of the sensor.
Full-bridge circuits typically have four sensing resistors that can be used to provide multiple compensation effects simultaneously. For example, two sensing resistors may be attached to the top side of the beam, and two may be placed on the bottom side of the beam. Thus, both lateral bending and axial strain can be filtered from the sensor output. In the alternative, all four sensing resistors can be placed on one side of the beam to provide increased compensation for lateral bending alone.
In all cases, the resistors used are separate and discreet. As a result, known strain gauges have a number of problems related to manufacture and installation. For example, despite the balancing effect of the bridge configuration, known strain gauges are subject to temperature variations that can cause inaccuracies in the sensor output. Due to the discreet nature of the resistors used, if a temperature gradient exists across the resistors, the temperature gradient may affect the output signal. Thus, the output signal will include variations unrelated to the parameter to be measured.
Similarly, mechanical damage to any of the resistors can occur. If, for example, one of the sensing resistors is scratched or plastically deformed through repeated loading, the resistance of the resistor may be artificially increased. The only crossover between the two current pathways is through the output signal detector. Consequently, when current shunts through the signal detector to reach the lower resistance current path, the sensor provides a false reading of the deformation of the member.
Furthermore, existing strain gauges are somewhat expensive and difficult to install. Each of the resistors must be made with some precision, or at least measured with accuracy, to ensure that the bridge is calibrated properly, or balanced at the appropriate deflection level. If the half-bridge or full-bridge configuration is to be used, each of the resistors must also be attached to the member at the proper orientation and respective location. In irregular or small members, it may be difficult to find adequate space for the sensing resistors. The resistors must also be connected in some way that will not interfere with the member or the sensor. Indeed, in many experiments involving strain gauges, simply attaching, connecting, and calibrating the sensing resistors often takes far more time than the actual testing.
Moreover, many strain gauges are ill-suited for applications in which opposing stresses are present in the same member. For example, if a beam is dually constrained, i.e., constrained at both ends, the simple bending stress distribution does not apply. A xe2x80x9cfixed-guidedxe2x80x9d beam, or a beam with one cantilevered end, and another end constrained to remain perpendicular to the cantilevered end, will undergo opposing stresses simultaneously when a force is applied perpendicular to the guided end. More specifically, since the fixed-guided beam bends in an S-shape, the side of the beam toward the origin of the force will be in tension toward the cantilever attachment and in compression toward the guided attachment.
As a result, a normal strain gauge configured to measure tension will provide varying output depending on where the gauge is positioned along the length of the beam. If the strain gage were placed over the center of the beam, resistive elements of the strain gage may cancel each other because one side of the center is in tension and the other is in compression.
Consequently, a need exists for an enhanced strain sensor that would resist the distortion caused by temperature gradients and mechanical wear. Furthermore, a need exists for a strain sensor that would be simple and inexpensive to manufacture, particularly in larger quantities. Yet further, it would be an advancement in the art to provide a strain sensor that would be comparatively simple to install on a member. Moreover, it would be an advancement in the art to provide a strain sensor that would provide a predictable output that could be readily correlated to strain when attached to the center of a dually-constrained member. Such enhanced strain sensors could find application in a wide variety of control systems.
One example of a control system in which such an enhanced strain sensor would be especially helpful is an automotive safety system. Such a system may include several safety elements designed to protect passengers in the event of an accident, such as seat belts and airbags. During operation of the automobile, it is desirable to control a number of parameters of the safety elements, such as the degree of tension in the seat belts, the deployment or non-deployment of the airbags, the volume of inflation gases used to inflate the airbags if deployment occurs, and the length of time the airbags remain inflated.
These parameters should preferably be selected intelligently and not arbitrarily. Thus, the automotive safety system requires data concerning various characteristics of the vehicle and passengers. For example, the tension in the seat belt can provide information concerning whether the seat is occupied, how large the occupant is, whether the occupant is an adult or a child in a car seat, whether the occupant is leaning forward, and what pressure the seat belt is exerting on the occupant. The weight carried by a seat (xe2x80x9cweight-in-seatxe2x80x9d) also can be used to detect the size of the occupant. The acceleration of the vehicle, including magnitude and direction, can be used to determine whether an accident is in progress or about to occur, and what the magnitude of the impact against the occupant will be.
The deformation of the vehicle frame can also indicate the severity of an accident, and thus, the impact force against the occupants. The velocity of the vehicle over time can also be used for impact measurement, or to determine the probable severity of potential future impacts. The pressure of various fluids could be measured to indicate the safety of the vehicle after an accident has occurred; for example, the pressure of gas in the gas tank could indicate whether a fuel leak has occurred. The temperature inside the vehicle could also be used as an indicator of fire in the passenger compartment of the vehicle.
Through the use of the appropriate sensors, decisions regarding seat belt tension, airbag inflation, and the like may be made more accurately and with greater confidence. Vehicle occupants will be better protected by an automotive safety system that receives accurate data and interprets the data intelligently.
Accordingly, a need exists for an automotive safety system equipped with sensors that function accurately and reliably over the comparatively long life of a vehicle. Such an automotive safety system should preferably be inexpensive to manufacture and comparatively simple to install. Furthermore, such an automotive safety system should preferably receive all the data necessary to ensure that safety elements are utilized to afford the maximum possible protection to occupants of the vehicle.
The apparatus of the present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available sensors. Thus, it is an overall objective of the present invention to provide a sensor that overcomes the problems of the prior art, and is particularly amenable to use in automotive safety systems.
To achieve the foregoing objects, and in accordance with the invention as embodied and broadly described herein in the preferred embodiment, a microstrain sensor is provided. In selected embodiments, the microstrain sensor provides functionality similar to that of a full-bridge Wheatstone Bridge circuit without the use of discreet resistors. Rather, a single expanse of film is used to conduct electric current between a plurality of conductors, preferably four, in contact with the film. The conductors may be arrayed about the film in a rectangular or diamond configuration to provide self-balancing effects similar to those of the conventional full bridge strain gauge. The conductors and film may rest on a comparatively thin insulator affixed to a deformable member, the deformation of which is to be measured.
Thus, for example, two input conductors may be positioned on opposite sides of a current-carrying portion of the film and connected to an input signal source. Two output conductors may also be positioned on opposite sides of the current-carrying portion, displaced from the input conductors. Thus, two separate conduction paths are created between the input conductors. The output conductors may be connected to an output signal detector, such as an ammeter, to detect current flow between the two conduction paths.
Such a configuration provides several benefits over conventional strain gauges using discreet resistors. In operation, the film provides a considerable current-carrying cross section, so that current can circumnavigate any scratches or other irregularities in the film that may otherwise detract from the accuracy of the sensor. The continuous array of potential current pathways assures that thermal affects and other potential unbalancing factors are mitigated. If the resistance of one portion of a conductive path is artificially raised, current can shunt around the irregularity without diverting to the other conductive path.
Additionally, although the relative placement of the conductors is important, the film must simply overlap all of the conductors such that a straight path between each of the conductors is provided. The film should preferably be a uniform thickness. However, if the film extends further on one side than on another, the operation of the sensor is not affected. Preferably, the film takes the form of a xe2x80x9cthick filmxe2x80x9d applied through a relatively rapid and simple process such as screening. The film may easily and inexpensively be applied, and the microstrain sensor may be installed in a very small space.
The microstrain sensor may take a wide variety of different configurations depending on the parameter to be measured. For example, the conductors may be placed side-by-side in a rectangular or square configuration on the deformable member to measure axial strain of the deformable member, such as may be caused by simple stretching, compression, or bending. In the alternative, the conductors may be arrayed in a diamond orientation on the deformable member to measure torsion, or tension/compression along an axis rotated 45 degrees from the longitudinal axis of the deformable member.
If acceleration is to be measured, the conductors may be arrayed to measure axial strain as described above. One end of the deformable member may then be affixed to a movable object, and the other end may be left free. Thus, acceleration of the object perpendicular to the film induces bending of the deformable member that is read by the microstrain sensor. A weight may be attached to the free end of the microstrain sensor to increase the gain, or sensitivity, of the microstrain sensor. Other desired parameters such as pressure and temperature could be similarly measured using the microstrain sensor, by arranging the deformable member in the proper fashion.
If strain of a dually constrained deformable member is to be measured, the microstrain sensor may be reconfigured somewhat. More specifically, the microstrain sensor may have a plurality of film sections positioned on either side of a central plane that divides the deformable member in half. The film sections may be joined to form a unitary film portion that crosses the central plane. The unitary film portion may then have a pair of input conductors disposed on either side of the central plane, and an output conductor between the input conductors. The portion of the input signal that reaches the output conductor then indicates the strain of the dually constrained deformable member.
Since the two film sections are on opposite sides of the central plane, one will be in tension while the other is in compression. Hence, the resistance of one will increase while the resistance of the other decreases. The positioning of the output conductor between the input conductors makes the opposing resistance changes have an additive effect on the output signal, rather than negating each other. Since there are two effective resistors, the resulting circuit is a half bridge.
Two such unitary film portions may be used to provide a full bridge configuration to yield greater output signal amplitude, temperature correction, or other benefits. Crossover circuitry may be used to connect opposite ends of the unitary film portions so that the two unitary film portions produce a combined, additive output signal.
The accuracy of any of the previously described microstrain sensors may possibly be enhanced by ordering the manufacturing steps such that a substantially uniform thickness of film is provided. For example, an insulator may first be applied on a deformable member to create a substantially flat, uniform surface on which the film can be placed. The film may be formed on the insulator with a substantially uniform thickness. The conductors may then be disposed on top of the film and attached in a way that does not significantly deform the film. Hence, a comparatively uniform resistance change may occur along the length of the film when the film is elongated or shortened.
An automotive safety system could beneficially use strain gauge sensors, and more particularly, the microstrain sensor of the invention, to enhance the safety of occupants of the vehicle. For example, microstrain sensors could be used to measure acceleration, deformation of the vehicle frame, the weight of the occupant, and tension on the seat belt.
Acceleration could be measured, for example, by affixing accelerometers incorporating the microstrain sensor, as described above, to the vehicle. The accelerometers could be affixed at multiple orientations to measure acceleration in multiple dimensions. Deformation of the frame could be measured by affixing microstrain sensors to the frame to measure axial or torsional strain in the frame. The weight of the occupant could be measured by affixing a microstrain sensor at one or more locations on the undercarriage of the occupant""s seat. Deformation of the undercarriage will occur in proportion to the occupant""s weight.
Tension in the seat belt could be measured in a number of ways. For example, a microstrain sensor could be affixed to some rigid portion of the seat belt, such as the latch plate, anchor plate, or buckle. In one embodiment, the buckle contains a microstrain sensor configured to detect the seat belt tension as well as the latched/unlatched status of the buckle assembly. The microstrain sensor may be affixed to a deformable member in the form of a leaf spring within the buckle. The buckle may have a lever arm that operates to hold the latch plate within the buckle, and to bend the leaf spring when the buckle is latched, thereby increasing the deformation read by the microstrain sensor to indicate the latched state of the buckle assembly.
When there is tension in the seat belt assembly, the latch plate pulls on the lever arm to cause elastic bending. The bending of the lever arm causes the lever arm to contact the leaf spring at a point further toward the free end of the leaf spring so that the deformation of the leaf spring is decreased. That deformation change can be read by the microstrain sensor to indicate that the seat belt tension has increased.
The microstrain sensors may all be connected to a single processing unit that processes the sensor data to determine the appropriate response of the safety elements of the safety system, such as the airbag system and the seat belt. Control wires may extend from the processing unit to the various safety elements of the vehicle. Thus, the operation of the safety elements can be optimized in response to the parameters received through the microstrain sensors.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.